Heat oil in a large, heavy-based pan. Cook the mushroom for 5 minutes, until softened. Add potatoes and pour in the stock. Bring to the boil, then lower heat to a slow simmer. Cook with lid on for about 15 minutes, until the potatoes are tender.

Use the hand blender stick or place into a food processor and blend it until all smooth, then return to pan. Reheat just before serving, and garnish with a sprig of coriander.